The present invention relates to a method of making a non-linear voltage dependent resistor containing a ZnO element including zinc oxide as its major component. Specifically, the present invention relates to a method of making a non-linear voltage dependent resistor by forming or coating a high insulating layer on the side surface of the element.
Heretofore, attempts have been widely made for effecting a side surface insulation to a non-linear voltage dependent resistor, for instance, including a ZnO (zinc oxide) element. One method uses a process which comprises the steps of sintering a shaped element, thereafter forming or coating an insulating layer of an organic substance, such as an epoxy compound, on the side surface of the sintered element.
Another method uses a process which comprises the steps of coating inorganic compounds on a shaped element which is not sintered, thereafter sintering the shaped element, and forming an insulating coating of glass or crystalline upon completion of the sintering step.
However, with the former method, the following drawbacks are pointed out. A first drawback is that the organic material, such as epoxy compound, to be coated and the element are not closely or densely packed. As a result, moisture is adsorbed into the element, thereby degrading its performance characteristics and reducing a high current withstand capability. A second drawback is that, because of a difference of thermal expansion between the element and the epoxy resin, there occur cracks in the epoxy resin coated on the side surface of the element due to thermal shock or impact, thereby degrading its performance characteristics.
A problem also arises with the latter method, wherein it is necessary to equalize a shrinkage ratio between the shaped element and the side surface insulating agent when sintering is effected. For this reason, there is actually employed the method of primarily sintering the element so that the formed element is shrunk by a desired volume ratio, thereafter coating an inorganic compound or mixture thereof on the side surface of the element, and sintering the element to form the insulating coating of an inorganic material. According to the last mentioned method, two sintering steps are required. As a result, the cost required for fabricating the resistor increases because of the fact that the fuel fee required for heat generation in this case is twice as large as that required for the single sintering step. Further, another drawback is apparent, in that the sintering apparatus is used tiwce, thereby reducing the durability thereof. Furthermore, with the latter method, there occurs the following unfavorable phenomenon in view of the non-linear voltage dependent characteristic: During sintering, Bi.sub.2 O.sub.3 vaporizes from the element. For this reason, there occur losses in uniformity of the thickness, which determines the degree of the non-linearlity of the element, of a grain boundary of Bi.sub.2 O.sub.3. Moreover, the sintering being carried out in this instance is effected as a liquid phase sintering due to a liquid phase of Bi.sub.2 O.sub.3. As a result, there occur losses in uniformity in the speed growth of ZnO crystal structure, thereby lowering the non-linear voltage dependent characteristic.
With both the abovementioned methods, a further drawback pointed out is that skilfulness in technique and a complicated device are required for making it possible to effect a control so that the thickness of the coating becomes uniform.